Generally, a tapered or conical helical antenna comprises an electrically conductive wire that follows a helical path on a lateral surface of a cone or frustum of a cone. With reference to FIG. 1A, a cone 10 is defined by a planar base surface 12 and a lateral surface 14 that is, in turn, defined as the locus of all straight line segments connecting an apex 16 and the perimeter of the planar base surface 12. The perimeter of the planar base surface is elliptical. In FIG. 1A, the perimeter of the planar base surface 12 is a circle, which is a particular type of ellipse having an eccentricity of zero. The circle has a radius of rb and a center 18. The cone 10 also has a rotational axis of symmetry 20 (hereinafter “axis 20”) that is defined as a line that passes through the apex 16 and the center 18 of the circular planar base surface 12. In FIG. 1A, the axis 20 is perpendicular to the planar base surface 12. The cone 10 is commonly referred to as a right circular cone with “right” referring to the angle between the planar base surface 12 and the axis 18 and “circular” referring to the shape of the perimeter of the planar base surface 12.
With reference to FIG. 1B, a frustum of a right circular cone 22 is illustrated. The frustum of a cone 22 is defined by a planar base surface 24, a lateral surface 26, and a planar top surface 28 that is parallel to the planar base surface 24. The perimeter of the planar base surface 24 is an ellipse and, in this example, a circle of radius rb with a base center 30. The perimeter of the planar top surface 28 is also an ellipse and, in this example, a circle of radius rt with a top center 32. The lateral surface 26 is defined as the locus of all straight line segments connecting the perimeter of the base surface 24 to the perimeter of the top surface 28 that, if extended, would pass through an imaginary apex 34. The frustum of a cone 22 has a rotational axis of symmetry 36 (hereinafter “axis 36”) that passes through the imaginary apex 34, the base center 30 of the circular planar base surface 24, and the top center 32 of the circular planar top surface 28. In FIG. 1B, the axis 36 is perpendicular to the planar base surface 24. Since the planar base surface 24 is circular, the frustum of a cone 22 can be characterized as a frustum of a right circular cone. The frustum of a cone has a height “h” that is the perpendicular distance between the planar base surface 24 and the planar top surface 28.
With reference to FIGS. 2A and 2B, a right circular conical helical antenna 40 includes an electrically conductive wire 42 and a ground plane 44. The electrically conductive wire follows a helical path on a lateral surface 46 of a frustum of a right circular cone 48 in which a tangent to a point on the helical path makes an angle relative to the plane of the planar base surface that remains substantially constant for each point on the path. Generally, the electrically conductive wire 42 follows a helical path that begins adjacent to the planar base surface of the cone and effectively terminates before reaching the apex of the cone. As such, the electrically conductive wire follows a helical path that begins adjacent to the planar base surface of the frustum of a right circular cone 48 and effectively terminates adjacent to the planar top surface of the frustum of a right circular cone 48. It should also be appreciated that a right circular conical helical antenna does not necessarily require a cone to support the electrically conductive wire. However, the electrically conductive wire must follow a helical path as the path would exist on the lateral surface of a frustum of a right circular cone to realize a right circular conical helical antenna. The ground plane 48 typically is situated parallel to the planar base surface of the cone 48 that is used to define the helical path of the electrically conductive wire 42. A coaxial cable 50 is used to connect the conical helical antenna 40 to a transmitter and/or receiver. More specifically, the center conductor of the cable 50 extends through a hole in the ground plane 48 and is operatively connected to the end of the electrically conductive wire 42 and the outer conductor of the cable 50 is operatively connected to the ground plane 44. In a conical helical antenna comprised of two or more conductive wires (i.e., a multi-arm or multi-filar conical helical antenna), a ground plane may not be needed, as can be appreciated by those skilled in the art. Further, multi-arm or multi-filar conical helical antennas employ a balun or feed network to feed each of the arms as known by those skilled in the art.
Conical helical antennas are designed to operate in one of two modes, a normal mode in which the maximum power density is perpendicular to the rotational axis of symmetry of the cone and an axial mode in which the maximum power is in the direction of the rotational axis. In conical helical antennas that operate in the axial mode, the height of the conductor or the perpendicular distance between the planar base surface and the planar top surface of the cone on which the conductor is modeled is directly related to the gain of the antenna, i.e., the greater the height, the greater the gain. Further, the circumference of the planar base surface of the cone on which the conductor is modeled is approximately equal to the wavelength of the low-end of the bandwidth of the antenna. Similarly, the circumference of the planar top surface of the cone on which the conductor is modeled is approximately equal to the wavelength of the high-end of the bandwidth of the antenna. Typically, the angle that a tangent to the helical path makes relative to the plane defined by the planar base surface of the cone on which the conductor is modeled is approximately 12°±4° for optimal axial mode operation.
With reference to FIG. 3, an embodiment of an array of right circular conical helical antennas 54 is described. The array 54 comprises two right circular conical helical antennas 56A, 56B that are substantially identical to one another. To elaborate, the antennas 56A, 56B respectively have electrically conductive wires 58A, 58B that respectively follow identical helical paths modeled on the lateral surfaces 60A, 60B respectively associated with substantially identical cones 62A, 62B. The axes 64A, 64B respectively associated with cones 62A, 62B are substantially parallel to one another. Planar base surfaces 66A, 66B of the cones 62A, 62B lie in a plane 68. A ground plane 70 is disposed substantially parallel to the plane 68 and serves as the ground plane for both of the conical helical antennas 56A, 56B. Associated with the array 54 is a phase center axis 72 that is a line defined by phase center points at each frequency of operation for the antenna.